Dietary fibers are edible carbohydrates, which are neither digested nor absorbed in the human small intestine and which have been obtained from food material by physical, enzymatic or chemical means and which have a beneficial physiological effect. In general, dietary fibers pass through much of the digestive system intact and may be totally or partially fermented by the intestinal microbiota.
Dietary fibers may be water soluble or water insoluble. Among the water soluble dietary fibers are fructans. Fructans essentially are polymers composed of fructose residues, ending or not with a glucose unit at what would otherwise be the reducing end. The linkage position of the fructose residues determines the type of the fructan. Linkage normally occurs at one of the two primary hydroxyls (OH-1 or OH-6), and there are two basic types of simple fructan: Inulin (the fructosyl residues are linked by β-2,1-linkages) and Levan (the fructosyl residues are linked by β-2,6-linkages). Fructans can be found in many plants as well as microorganisms, where they are stored as a form of energy. For instance inulin is produced in particularly high quantities in chicory roots.
Industrial production of inulin from for instance chicory root typically involves extraction by hot water. This method produces an extract rich in inulin. However free sugars (for instance glucose, fructose and sucrose) are also co-extracted. An inulin rich extract typically contains about 70-85 wt % inulin and 5-13 wt % free sugars, along with 10-17 wt % of other impurities (e.g. salts, proteins, etc.) based on dry matter. The exact composition of for instance chicory inulin rich extracts however varies and for instance depends on the growing conditions, harvest date, variety of chicory, etc.
The method for purifying the inulin rich extract typically contains several steps, including for instance solid/liquid separation, ion exchange, activated carbon filtration, etc. in which the majority of impurities are removed, and inulin rich composition is obtained. However, free sugars which have a structure and/or chemical characteristics very similar to that of fibers such as fructans, in particular inulin, most often are not eliminated from the inulin rich composition, and these can represent between 6 and 16% of the dry matter (based on dry matter for instance 1-2 wt % glucose, 1.5-7 wt % fructose, and 3.5-7 wt % sucrose in inulin rich compositions from chicory roots).
Although physico-chemically and structurally similar to fibers, such as fructans, these free sugars are however distinguished by their nutritional properties, in view of their digestibility which thus provides a high caloric value as opposed to fibers. High free sugars impurities in dietary fiber compositions therefore pose a problem for instance for diabetics. From this point of view, it is highly advisable to minimize the contents of such free sugars, in the inulin rich composition. Moreover, from a technical point of view, often the industrially produced fibers are provided to customers in the form of syrups or powders. In the latter case the last step of the process may involve spray drying. The effectiveness of this well-known technique decreases as the content of free sugars increases, the latter being more “difficult to dry” because of their relative hygroscopicity (mainly fructose), such that increasing the elimination of free sugars, and in particular fructose, prior to drying not only has nutritional, but also technical advantages.
There are several ways of separating free sugars from the fiber extracts, for example fractional precipitation based on the relative solubility or chromatography. However, these physicochemical separation techniques are very expensive and have a limited scale performance. The fractional precipitation technique is used industrially for the production of fibers that contain reduced free sugars amounts. This technique is based on the differential solubility of carbohydrates molecules of different molecular weights. The industrial scale chromatography allows separation of free sugars on different ranges of fiber types and the efficiency is higher than in the case of fractional precipitation, but remains low. In any case it is not possible to separate the free sugars without conceding to a loss of fibers.
In view of the above, there is still a need to develop alternative or improved methods for eliminating free sugars from dietary fiber compositions, in particular fructan compositions, such as inulin compositions. It is accordingly one of the objects of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.